The present invention relates to a device and an LSI for executing a plurality of signal processing tasks, and particularly to a technique effective in suppressing overhead associated with task switching to priority processing.
In digital devices for processing images such as cellular phones, digital cameras, and car navigation systems, there is a tendency that image resolutions become higher and functions become more sophisticated and complex; accordingly, LSI etc. for performing signal processing on stream data such as images are required to execute a plurality of image processing tasks. At this time, in signal processing devices, task switching associated with a priority processing request occurs, and overhead associated with the task switching is required to be suppressed. The task switching associated with a priority processing request refers to an operation for receiving a request for priority processing of a further task during the execution of a current task, suspending the currently running task, executing the priority task, and returning to the original task after the completion of the priority processing. In general, a method called a context switch is used for task switching. The context switch is a method for saving the value and state of a register which is information about the progress of the running task at the time of task switching, switching tasks to execute priority processing, and restoring the saved information to resume the original task after the completion of the priority processing. Since the task of signal processing of stream data such as images deals with an enormous amount of information in comparison with ordinary software, it takes a great deal of time to save the progress of the running task into a memory and return, which disadvantageously increases overhead.
In particular, image processing is often performed in units of frames, and notification such as an interrupt is made every time the image processing of one frame is completed and acts as a trigger so that a system performs various kinds of processing. Higher resolution (bigger size) of processed images and complication of image processing are further increasing processing time and data amount per frame. Accordingly, it takes a greater deal of time to save the progress of the running task of image processing into a memory and return, which disadvantageously further increases overhead.
Japanese Unexamined Patent Publication No. 2009-86789 describes in paragraph 0007 that in image processing, due to a large amount of data to be processed in a collective manner (e.g., images for one screen), a context switch cannot be performed (very inefficient if not impossible). For multitask image processing, a plurality of image processing devices are provided and operated in parallel for scheduling for preventing the occurrence of context switching.
Japanese Unexamined Patent Publication No. 2006-197548 discloses a technique for performing display on a priority basis in compression processing in an image pickup device in which image data acquired by shooting is compressed by an image processing unit and stored. In the device, the image processing unit is not used for display; therefore, by stopping the image processing unit during the display on a priority basis, the middle state of processing is maintained, and after return from priority processing, the image processing unit can be resumed as it is. That is, a context switch does not occur in the image processing unit.